1. Field of Invention
The present invention claims an underground annular blowout preventer, which belongs to the technical field of underground blowout prevention.
2. Description of Related Arts
In the task of oil exploration and oilfield development, well drilling is very important. Looking for and proving oil and gas bearing structures, obtaining industrial oil output, verifying oil and gas bearing areas and reserves of the proved oil and gas bearing structures, obtaining geological data and development data related to the oil field, and extracting the crude oil from underground are all completed through well drilling. Well drilling is a very important part in exploring and exploiting oil and gas resources and an important method of exploring and exploiting oil.
During the well drilling process, when there is oil-gas reservoir, if the bottom-hole pressure is lower than the formation pressure, the formation fluid will enter the well. If a large amount of formation fluid enters the well, well kick, blowout, or even fire may result and cause a major accident. Therefore, it is important to take effective measures to control the pressure in the oil-gas well during well drilling process. However, it is very hard to predict the pressure of the high-pressure oil and gas layer, especially when drilling in the new blocks. As a result, high pressure oil and gas from the stratum may accidentally enter the well during well drilling process. In addition, well kick and blowout accidents occur in the following three states: during the drilling process, lifting and lowering the drill pipe, and emptying the well. The probability of well kick in an empty well is very low, and the probability of well kick or blowout during process of lifting and lowering the drill pipe can be effectively reduced by controlling the velocity of lifting and lowering the drill pipe to avoid overly large suction forces. Because it is impossible to predict the pressure of a high pressure oil and gas layer during drilling process, the probability of well kick or blowout during this process is the highest.
When sign of well kick or blowout is discovered, rapidly shutting in and killing the well are highly effective operations in reducing blowout accidents. The traditional well shut-in operation is as follows (soft shut-in is usually used in China): after discovering well kick, shutting down the blowout preventer after opening the throttle valve, such that the water hammering action to the well head equipment and annulus is prevented; however, the amount of stratum fluid that entered will be relatively large because the time for well shut-in is long. The extra stratum fluid can cause additional pressure to the well head equipment and stratum, which makes well killing more difficult.
Presently, most blowout preventers used for the well drilling process are mounted at the well head. During the well drilling process, an arrow-shaped check valve is mounted on the drill pipe near the drill to avoid a reflux of fluid inside the drill pipe. The blowout preventer at well head is usually used to seal the annulus during drilling process and only used for well sealing when it is necessary to cut the drill pipe. When a sign of well kick is discovered, the operator turns on the control system to finish the well shut-in operation so as to avoid a blowout accident. This kind of operation relies on the experience and responsibility of the operator. However, if the operator lacks of the experience or is negligent during work, the sign of well kick or blowout may not be discovered in time, and a major accident may occur. After a well shut-in operation, the time required for well killing a well with heavy mud on the traditional well head blowout preventer is long, which results in more bottom fluid entering the well and increases the annular pressure difference, and thus it is more difficult to control blowout. Therefore, if an underground annular blowout preventer can be invented and lowered into the well together with the drill pipe, when the sign of underground well kick or blowout is discovered from the ground, the operator can operate the underground annular blowout preventer to seal the annulus and the check valve near the drill which can effectively seal the inner side of drill pipe. Thus the inner side of drill pipe and annulus over the annular preventer are in communication with each other after sealing, which facilitates the subsequent well killing operation with heavy mud. By using a check valve near the drill and an early-phase well kick pre-warning system together with the underground annular sealer, the well control principle of early-discovery and early-treatment can be realized. This operation is easier to be executed than the common well control operations.
The Chinese invention patent application which was published on 23 Jun. 2010 and named as A well blowout preventer mounted in a drilling column and automatically controlled in a well, with application number of 200910263415.7 and publication number of CN101748984A, discloses an underground preventer which can prevent blowout during well kick to a certain extent. However, it has a complex structure and many components; therefore, it is complex to install, replace, and has an accordingly high cost. After being used to prevent a blowout, the preventer can be recovered and reused from the original structure through the spring when there is no pressure difference, but if there is a pressure difference inside and outside the preventer, the preventer most likely cannot be recovered for reuse because elastic force of the spring is less than the pressure difference. After being used multiple times, the spring can easily be broken; if the spring is broken when the blowout preventer is in use, the blowout preventer cannot be recovered and taken out, and thus the blocking caused thereby brings large inconveniences to the subsequent operations.
The Chinese invention patent application which was published on 23 Aug. 2010 and named as A mechanical underground all-in-one blowout preventer, with application number of 201010148874.3 and publication number of CN101812981A, discloses an underground all-in-one blowout preventer which can prevent a blowout during well kick to a certain extent. However, it has a complex structure and uses a shearing pin to seal; the pin is used to connect the upper and lower drilling tool and transfer the torque in the well drilling process, but the pin can be broken after undertaking long periods of variable shearing forces and variable torque because the drill pipe vibrates in drilling process. Therefore, the reliability of the blowout preventer is not high since it is easy to cause an incorrect operation and due to an unexpected pin break a series of underground accidents can occur. Moreover, when it is necessary to seal the high drilling pressure, the lower drill is required to cut the pin. However, the lower drill components can only sustain a certain drilling pressure, which means that the process of cutting the pin can easily damage the lower drill components and cause an underground accident. The invention patent application has one more problem. Even if all the operations before sealing the annulus are normal, if the annulus sealing is not complete, the high pressure stratum oil and gas inside the annulus can enter the drill pipe through the flow guiding hole, which makes the arrow-shaped check valve fail to seal the inner side of the drill and makes the well control more difficult. Furthermore, after the pin is cut, well drilling has to be stopped and it is mandatory to take out the drill pipe and replace or maintain the all-in-one blowout preventer; that is, the blowout preventer can only be used once after being lowered to the well, which is unfavorable for reuse.
The Chinese invention patent application which was published on 2 Feb. 2010 and named as A lifting valve type underground inside-outside integrated blowout preventer, with application number of 200910312467.9 and publication number of CN101718181A, discloses an underground all-in-one blowout preventer which can prevent a blowout during a well kick to a certain extent. However, it has a complex structure and needs to be driven by a motor. Some problems with this invention are that it is difficult to install the motor, and a high capacity power source is required in the narrow underground space. There is doubt that whether the power provided by the motor can meet the minimum power requirements required by the sealing and unsealing processes (especially when the pressure difference is high). Therefore, the safety and reliability of the patent are not high, and it is not very practical to be used in actual well drilling.